In phase locked loops wherein a voltage controlled oscillator is controlled to phase lock onto a frequency spaced from a reference frequency by a specific offset frequency, it is well known in the art that the VCO can lock onto a frequency above the reference signal by the offset frequency (upper sideband) or below the reference signal by the offset frequency (lower sideband). It is, of course, essential that the VCO lock onto the desired sideband and toward this end elaborate and critical single sideband mixing techniques are used in many circuits. Single sideband mixing techniques require relatively complicated RF hardware and relatively critical parameters depending upon sideband suppression, carrier suppression, etc.
In U.S. Pat. No. 4,009,448, entitled "Phase Lock Loop For A Voltage Controlled Oscillator" issued Feb. 22, 1977, apparatus is disclosed wherein a phase locked loop can be locked onto a desired sideband without using the prior single sideband mixing techniques. This apparatus utilizes inphase and quadrature mixers to sense the sideband to which the VCO is locked. In the patent the apparatus is locked to the upper sideband and, if the VCO locks on the lower sideband, a sweep circuit is activated to unlock the VCO and cause it to continue sweeping. This apparatus requires relatively complicated 90.degree. phase shifting circuitry. Also, the bandwidth of the apparatus is somewhat reduced because of the lock sensing apparatus.